Mandrill reproductive variance

06 Apr 2006

Joanna Setchell and colleagues (2005) present observations on the sexual competition and reproductive success in mandrills. For a quick primer on mandrill social interactions:

Mandrills (Mandrillus sphinx, Cercopithecidae) live in multi-male, multi-female groups, and are one of the most sexually dimorphic species of land mammal, typifying the sex differences that prompted Darwin to develop his theory of sexual selection. Male body mass is 3.4 times that of females (Setchell et al. 2001), and male canine teeth measure 44 mm, versus 10 mm in females (Setchell and Dixson 2002; Leigh et al. 2005). Adult males also possess a variety of exaggerated secondary sexual adornments, including brightly coloured skin on the face, rump and genitalia; boney supra-maxillary swellings; a yellow beard; a long cape of hair and an epigastric fringe of white hair (Hill 1970). This adult sexual dimorphism is reflected in patterns of growth and development: while females reach adult size at the age of 7 year, males do not attain adult size and appearance until 9-10 year (Setchell et al. 2001; Setchell and Dixson 2002). Differences between males and females are thought to have evolved due to intense male-male competition in this species (Wickings and Dixson 1992b; Setchell and Dixson 2001), although female choice for large, ornamented males may also be involved (Setchell 2005).

The study kept track of reproduction and mortality in a captive population of mandrills. So it doesn't exactly replicate wild population dynamics, but the results are still striking in showing strong difference between males and females.

Mortality was generally low, but a marked sex difference was observed (Fig. 1; log-rank test statistic=11.44, df=1, p<0.001). The two sexes were indistinguishable until the age of 4 year, but sex differences became marked at 6 year. Of 111 females, only eight disappeared (7%): seven before 5 year, and one after 19 year. Females lived to 22+-1 year (95% CI 21-24 year, median survival could not be calculated for females, due to the small number of disappearances), although the data were limited to 25 year, meaning that females may survive longer than this. Male survival was 14+-1 year (95% CI 13-16 year, median 17 year); no male lived longer than 20 year. Males that disappeared prior to reaching adulthood (n=14) simply disappeared from one day to the next. However, of the 10 adult males that disappeared, four were seriously wounded prior to disappearance (likely as a result of male-male combat); three of these were alpha male at the time.

This is in captive enclosures totalling less than 10 ha. In other words, the males are pretty rough on each other. And it has results: reproductive variance is many times higher in males than in females -- up to 10 times higher including males who died without reproducing. Female reproductive variance may be understated by the study, since there is little or no predation, little infant mortality, and the population is growing -- all these should tend to make females more equal in their reproductive success.

But the age distribution of reproduction is the most impressive result:

Reproduction vs. age in mandrills, from Setchell et al. (2005)

Overwhelmingly, alpha males have almost all the offspring, and they fall into a limited class of ages. They have to be big enough (and experienced enough), and they have to be healthy and strong enough. And when they aren't, they're out.

Nine males attained top (alpha) rank during the study period. Males gained alpha-status between the ages of 9 and 14 year, with the exception of a 4-year old who had no competing adult male. Males that survived longer were thus significantly more likely to become alpha (logistic regression: alpha vs. non-alpha, B=0.38, standard error=0.12, Wald statistic=10.66, df=1, p=0.001, Exp(B)=1.46). However, of the 22 males that reached adulthood and could potentially attain alpha status, only nine (41%) did so, while only four of ten males (40%) that reached 15 years (older than the oldest male that became alpha) became alpha during their career. Tenure as dominant male varied between 1 month and 6 year (mean 34+/-9 months, median 24 months).

Attaining alpha rank had a clear influence on reproductive output (Fig. 5), and alpha males sired 85% of offspring (163 of 193 resolved paternities). Alpha males sired up to 13 infants in any one year, versus a maximum of four infants for non-alpha males.

John Hawks is the Vilas-Borghesi Distinguished Achievement Professor of Anthropology at the University of Wisconsin—Madison. I work on the fossil and genetic record of human evolution (About me).

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